1. Field of the Invention
The present invention relates to magnetic resonance (MR) imaging, and more particularly relates to a method and apparatus for more quickly and easily visualizing interventional devices, such as metal-tipped catheters, especially during movement of such devices during an interventional procedure.
2. Description of the Related Art
Known methods of visualizing the movement of catheters by MR imaging typically rapidly acquire data from a thin slab (“slice”) of tissue. A ferromagnetic material (“metal”) typically located near the distal end (tip) of the catheter causes a local distortion of the otherwise homogeneous magnetic field used by the MR imaging apparatus. If the metal part of the catheter is in or sufficiently near the slice that is currently being imaged, that local distortion of the field causes an image intensity near the position of the metal to be either brighter or darker than normal. This effect of intensity distortion has been used, in a general manner, to help find the catheter as it is moved through the body. For the purpose of visualizing the distal end of the catheter, it is helpful to construct the catheter in such a manner that the tip creates more distortion than the remaining parts. It is also generally known that the size and appearance of the intensity distortion artifact is affected by the precise sequence and timing of the radiofrequency pulses, data collection periods, and magnetic field gradients (i.e., the “pulse sequence parameters”) of the MR apparatus. For example, the pulse sequence parameters can be adjusted to produce a strong intensity distortion in response to the metal in the catheter, which spreads over a relatively large spatial area, or a weak intensity distortion that is confined to a small region of the image.
One sequence parameter that affects the intensity of the distortion is the so-called “echo time” or TE, the time between radiofrequency excitation and acquisition of the signal. Another parameter that affects the intensity of the distortion is the strength of the magnetic field gradient applied during the data acquisition (the so called “readout gradient”). It is not common to alter this gradient amplitude by itself, as this would undesirably change the field of view of the reconstructed image along one axis. Instead changes to this gradient amplitude are commonly made in conjunction with changes to several additional sequence parameters, and the net effect is referred to as a change in “receiver bandwidth.” Thus, as used herein, the term pulse sequence parameters includes not only the various values of the applied pulse sequence itself, such as the TE duration and readout gradient filed strength, but also the corresponding MR signal processing parameters required to develop an appropriate image in response thereto, such as coordinated analog or digital low pass filtering with an appropriate signal sampling rate, etc.
Under commonly-used conditions, pulse sequences with longer echo times and lower strength readout gradients yield images with increased distortion.
A disadvantage of adjusting the pulse sequence parameters, such as TE or the receiver bandwidth, so as to produce a spatially large distortion is that it may be difficult to localize the exact position of the tip of the catheter. On the other hand, an advantage of this large distortion is that, even if the catheter does not lie precisely in the selected image slab, some distortion will be visible if the catheter is sufficiently near the selected image slab.
The present inventor realized that this distortion effect may in some cases be helpful, because it is not always possible, prior to an image acquisition, to select an image slab that precisely includes the catheter position. A large spatial distortion could allow the catheter tip to be located even when the slab position is imperfect, increasing the ease of tracking the catheter tip as, for example, it is threaded through a vessel.
Even furthermore, it would be advantageous for the physician to have images showing both spatially large and spatially small distortions. To visualize the movement of the catheter, it would be advantageous to obtain multiple images of the same slab of tissue, one after the other, in rapid succession. However, the physician typically does not have time to change pulse sequences or type in new pulse sequence parameters during an interventional procedure, nor have the ability to easily select which images to display.